The present invention relates to aircraft multiple disc brake assemblies using either conventional or carbon composite frictional material.
Aircraft multiple disc brakes are designed to achieve a given maximum stopping distance under both favorable, dry, and unfavorable, wet, conditions. In many environments, especially for commercial aircraft, the brakes must be capable of producing sufficient torque to stop the airplane from high speed in a fixed maximum allowable distance, as in a rejected-take-off (RTO). Generally, the friction characteristics of the friction material in the brake are poorer when wet than when they are dry and usually the brake pressure is fixed to a maximum allowable amount. These design considerations thereby require the brake piston size to be larger than necessary for dry conditions in order to produce the required braking torque under wet conditions. On the other hand, there are maximum torque limits set for brake performance usually because of structural limitations of the landing gear strut or strut mountings. In some cases these different torque requirements are incompatible because the brake with pistons large enough to provide sufficient torque when the friction material is wet, provide too much torque when the friction material is dry. This is particularly important for brake assemblies utilizing carbon composite friction surfaces which exhibit significantly different friction characteristics under unfavorable and favorable conditions, having poor and good coefficients of friction, respectively.
To solve this dry-over torque condition, various torque-limiters were developed. Reference is made to U.S. Pat. Nos. 2,764,263 and 2,781,871 for typical systems known in the brake art that utilize "pressure dumping" techniques. "Pressure dumping" systems are also common to vehicle brake anti-skid systems. "Pressure dumping" occurs when a torque overload condition is detected such that the pressurized brake fluid, which is acting upon the brake pistons to produce the torque, is partially vented, i.e. "dumped", to a lower pressure reservoir or sump. This "dumping" lowers the pressure acting upon the brake pistons, thereby reducing the braking torque.
"Pressure dumping" torque limiting systems have several serious disadvantages. One is that when the system dumps fluid upon signal from the torque overload sensor there is no lower limit to how low the pressure can drop. If for example, the system response is so slow that the brake pressure drops to near zero, the brake does not produce an average torque sufficiently high to make stop distance requirements and the aircraft experiences large oscillations in deceleration rate, making for an uncomfortable stop. Secondly, overdumping of brake fluid is wasteful of fluid in a displacement limited system and the overdumping slows down response time of the system. In addition, the pressure reduction system is not self-contained in the brake, thus complicating system installation and maintenance.